1. Field of the Invention
The present invention relates to an NMR probe used when nuclear magnetic resonance (NMR) measurements are made. This NMR probe permits a sample tube to be introduced and ejected from the probe while the probe is kept mounted in a magnetic field generator. The sample tube is passed through a tubular sample tube passage between a sample tube insertion port and a sample tube support by a gas stream.
2. Description of Related Art
An NMR spectrometer is an analytical instrument for detecting a signal arising from atomic nuclei having spin magnetic moments by applying a static magnetic field to the nuclei to induce the spin magnetic moments to thereby produce a Larmor precession and irradiating the nuclei with RF waves having the same frequency as the precession to bring the nuclei into resonance.
In an NMR spectrum of a sample in solid phase, interactions (such as dipolar interactions which are nullified by rotational Brownian motion in a solution) manifest themselves directly and so the spectral linewidth broadens extremely, thus obscuring chemical shift terms. One of the methods which overcome this undesired phenomenon and give rise to sharp solid-state NMR spectra was discovered by E. R. Andrew in 1958 and is known as MAS (magic angle spinning). In this method, the sample tube is tilted at an angle of about 54.7° from the direction of the static magnetic field B0 and spun at high speed. Thus, anisotropic interactions can be removed and chemical shift terms can be extracted.
An NMR probe for implementing the MAS method is hereinafter referred to as the MAS probe and subjected to NMR measurements by being inserted in a slot-like measurement space within a magnetic field generator typified by a superconducting magnet system. Generally, the magnetic field produced by a magnetic field generator is either parallel or perpendicular to the measurement space. A magnet producing the former type of magnetic field is known as a solenoid-type air-core magnet. A magnet producing the latter type of magnetic field is known as a split-type air-core magnet. On the other hand, in a MAS probe, a sample tube holding a sample therein is placed on an axis of rotation tilted relative to the magnetic field on a sample tube support. This sample tube support acts to support the sample tube during measurements and to accurately determine the posture and motion of the sample tube. The sample tube is allowed to be introduced and ejected only from a certain direction.
Therefore, under the condition where the MAS probe has been attached in the magnetic field generator, it is difficult to introduce a sample tube from outside a sample tube support having its axis of rotation tilted relative to the magnetic field. Accordingly, it is customary to remove the MAS probe from the magnetic field generator in order to introduce a sample tube into the sample tube support.
However, when the MAS probe is used, the magic angle needs to be accurately adjusted in advance by an NMR measurement employing a reference standard. When the MAS probe is installed to or detached from the magnetic field generator for exchange of the sample, the adjusted value deviates from its correct value. Therefore, the adjustment must be made each time. Such a mounting and detaching work must be done manually. This causes human errors. In addition, this greatly deteriorates the efficiency of NMR measurements.
In an attempt to solve these problems, top-loading type MAS probes have been proposed, for example, as shown in JP-UM-A-57-59356 and U.S. Pat. No. 7,282,919. A MAS probe of the top-loading type permits a sample tube to be loaded from outside of a magnetic field generator that is spaced from a sample tube support. With the MAS probe of the top-loading type, a sample can be exchanged without mounting or detaching the MAS probe to or from the magnetic field generator.
The structure of a solid-state NMR spectrometer including a top-loading type MAS probe is schematically shown in FIG. 1, the probe being indicated by numeral 3. In FIG. 1, a superconducting magnet 100 is composed of a heat-insulating container 1 for accommodating a refrigerant and a superconducting solenoid coil C that is maintained at low temperature by the refrigerant. The heat-insulating container is provided with a through-hole 2 extending through the coil C. In this way, access to the magnetic field is provided through the through-hole 2 being an access space that is a slot elongated in the direction of depth compared with the width.
Referring also to FIG. 2, the MAS probe 3 is inserted in the through-hole 2, and is composed of a sample tube support 50, a sample tube insertion port 20 formed in the upper end of the through-hole 2, a sample tube passage 10 connecting together the insertion port 20 and the tube support 50, and a direction converter 40 mounted at a non-end position in the sample tube passage 10. The outer periphery of a sample tube 60 (see FIG. 2) is supported by an accurate gas bearing within the sample tube support 50 and on an axis of rotation tilted relative to a static magnetic field H produced by the superconducting coil C. During measurements, the sample tube is spun at high speed.
FIG. 2 more particularly shows the structure of the MAS probe 3 shown in FIG. 1. In FIG. 2, a sample to be investigated is sealed in the sample tube 60 that is a roughly circular cylinder. The sample tube 60 having the sample sealed therein is inserted in the sample tube insertion port 20, transported to the sample tube support 50, and subjected to a measurement. After the measurement, the sample tube 60 is withdrawn from the sample tube support 50 and taken out of the sample tube insertion port 20.
The tubular sample tube passage 10 consisting of passage portions 10a and 10c extend between the sample tube insertion port 20 and the sample tube support 50 to connect together port 20 and tube support 50. The sample tube 60 can move between the sample tube insertion port 20 and the tube support 50 by moving substantially parallel to the axis of the cylindrical interior of the sample tube passage 10.
In order to introduce the sample tube 60 into the sample tube support 50 from the direction of axis of rotation, it is necessary to convert the direction of the axis of the cylindrical interior of the sample tube 60 that descends vertically from the sample tube insertion port 20 into a direction parallel to the axis of rotation of the sample tube support 50 to permit introduction into the sample tube support 50. For this purpose, the direction converter 40 is mounted near the sample tube support 50.
After undergoing a measurement, the sample tube 60 is expelled from the sample tube support 50 toward the sample tube insertion port 20 by gas flowing through the sample tube passage 10. The gas is supplied from a high-pressure gas generator 230 connected to the bottom of the sample tube support 50 via a pipe 200 under control of a valve 210.
During the ejection of the sample tube, the sample tube 60 is forced toward the direction converter 40 by the gas ejected from the bottom of the sample tube support 50. The direction converter 40 has a curved inner wall surface to change the direction of the sample tube 60. As the sample tube 60 enters the direction converter 40, the sample tube abuts against the curved inner wall surface of the converter 40 and, thus, the tube moves along the inner wall surface. Consequently, the direction of motion is changed into a perpendicular direction. Then, the sample tube 60 is raised to the position of the sample tube insertion port 20 by the gas pressure. During this process, a flow rate of gas sufficient to move the sample tube up to the sample tube insertion port 20 against gravity is supplied from the bottom of the sample tube support 50 via the valve 210.
On the other hand, when the sample tube 60 is introduced, it is moved from the sample tube insertion port 20 toward the sample tube support 50 mainly by gravity, the sample tube support 50 having a sample tube-spinning function as described later. To avoid the tube 60 from dropping abruptly, contrivances are adopted. For example, the sample tube 60 is floated by the gas pressure when the tube is inserted into the sample tube insertion port 20 and then the gas pressure is lowered gradually so that the tube descends slowly.
In this way, with the top-loading type MAS probe, the investigated sample can be exchanged while the probe is kept mounted in the magnetic field generator. This dispenses with readjustment of the probe concomitant with a sample exchange. Hence, the NMR spectrometer can be used efficiently. Furthermore, different samples can be continued to be measured while maintaining the sufficiently adjusted condition achieved using a reference standard and, therefore, accurate NMR spectra can be obtained easily.
With the conventional MAS probe as described above, the sample tube 60 is pushed out of the sample tube support with strong force during ejection of the sample tube. Therefore, the outer periphery of the sample tube 60 and the inner bearing of the sample tube support 50 are strongly rubbed against each other. This wears away the outer periphery of the sample tube 60 and the inner bearing of the sample tube support 50. Generally, a gas bearing fabricated to an accuracy of the order of micrometers is used as the bearing of the sample tube support 50. If the bearing is worn slightly, the performance of the sample tube support 50 will deteriorate.
Similarly, when the sample tube 60 is ejected, the sample tube 60 violently collides against the inner wall of the direction converter 40. The direction of the tube is then converted by motion bound to the inner wall, i.e., while pressed against the wall. Because of the collision and friction occurring at this time, the outer periphery of the sample tube 60, especially the turbine blades for driving the MAS probe 3, is damaged or greatly worn away, resulting in a decrease in the lifetime of the tube. If the sample tube 60 is worn, the MAS probe 3 will malfunction. When the sample tube 60 is inserted or withdrawn, the sample tube passage 10 will be clogged up.
For instance, the turbine blades of a sample tube 60 of a complex shape as shown in JP-A-2003-177172 show excellent aerodynamic performance in realizing high-speed spinning, but the mechanical strength is low because the complex and delicate structure of the curved turbine blades is exposed from the outside of the sample tube 60. A sample tube 60 having such turbine blades is subject to damage and wear in the conventional MAS probe 3 and has been difficult to use in practical applications.
Furthermore, the aforementioned collision and wear will wear away the inner wall of the direction converter 40, resulting in troubles.